1. Field of the Invention
The present invention relates to a liquid crystal diffraction lens element capable of switching a focal point, and to an optical head device employing the liquid crystal diffraction lens element for writing and/or reading (hereinafter referred to as writing/reading) an information to/from an optical recording medium such as an optical disk.
2. Discussion of Background
Heretofore, an optical head device for writing/reading an optical recording medium such as an optical disk, carries out writing/reading an information to/from optical disks having different cover thicknesses such as CD and DVD. Here, between optical disks having different cover thicknesses, the depths of their information recording planes for recording an information, from their disk surfaces (hereinafter simply referred to as the depth of information recording plane) are different. In order to enable writing/reading of a plurality of optical disks having information recording planes of different depths, an optical modulation element for switching a focal length is employed (refer to e.g. JP-A-9-230300; hereinafter referred to as Patent Document 1.).
Here, the optical modulation element disclosed in Patent Document 1 comprises a pair of transparent substrates, a liquid crystal sandwiched between these transparent substrates, an optical member having a Fresnel lens shape provided at an interface between the liquid crystal and one of the transparent substrates, and transparent electrodes for applying a voltage to the liquid crystal. The optical modulation element is configured to switch the voltage applied to the liquid crystal to switch alignment direction of the liquid crystal, to thereby switch the refractive index of the liquid crystal for incident light to switch presence and absence of Fresnel lens function of the optical member.
For the optical head device, usually, light polarized in a predetermined direction is used as incident light, and switching of the refractive index is carried out by making the alignment direction of the liquid crystal agree with the polarization direction of incident light. The refractive index of the liquid crystal is switched so that it agrees or differs from the refractive index of the optical member having a Fresnel lens shape before and after switching of a voltage applied to the liquid crystal.
Further, in order to improve recording density of an optical disk, an optical head device employing a laser diode emitting light of wavelength 405 nm, has been proposed. For the optical head device using this wavelength band, roughly two standards have been proposed. They are the Blu-ray standard which uses a cover layer of 0.1 mm thick made of transparent resin covering an information recording layer of an optical disk and uses an objective lens of NA (numerical aperture) 0.85, and the HDDVD standard which uses a cover layer of 0.6 mm thick and uses an objective lens of NA 0.65. Further, in each of the Blu-ray standard and the HDDVD standard, in order to increase information volume in each optical disk, optical disks having multi-layer information recording layers have been developed.
However, in such a conventional optical head device, since the liquid crystal in the optical modulation element is uniformly aligned, there has been a problem that for light polarized in a direction in which the liquid crystal shows the ordinary refractive index, the refractive index of the liquid crystal can not be switched by switching an applied voltage. In an optical head device for writing/reading e.g. DVDs or high density optical disks (the Blu-ray standard or the HDDVD standard), in which a polarizing optical system is generally employed, it is possible to switch focal length of only one of outgoing light being light toward an optical disk and returning light being light reflected by the optical disk. For this reason, heretofore, in order to enable switching of focal lengths of both of outgoing light and returning light, an optical modulation element is provided for each of outgoing light and returning light, which causes e.g. increase of the size of optical head device, complication of driving, or increase of cost.
Further, in order to establish compatibility between the Blu-ray standard and the HDDVD standard, an element capable of variably changing a focal point depending on the difference of cover layer thickness, has been desired.
Further, an element for variably changing a focal point at a time of writing/reading layers of multi-layer optical disks of each standard, has been desired.